Proteolytic processing is a major form of post-translational modification that occurs when a protease cleaves one or more bonds in a target protein to modify its activity. This processing may lead to activation, inhibition, alteration or destruction of the protein's activity. Many cellular processes are controlled by proteolytic processing. The attacking protease may remove a peptide segment from either end of the target protein, but it may also cleave internal bonds in the protein that lead to major changes in the structure and function of the protein.
Proteolytic processing is a highly specific process. The mechanism of proteolytic processing varies according to the protein being processed, location of the protein, and the protease.
Proteolytic processing can have various functions. For instance, proteolysis of precursor proteins regulates many cellular processes including gene expression, embryogenesis, the cell cycle, programmed cell death, intracellular protein targeting and endocrine/neural functions. In all of these processes, proteolytic cleavage of precursor proteins is necessary. The proteolysis is often done by serine proteases in the secretory pathways. These proteases are calcium-dependent serine endoproteases and are related to yeast and subtilisin proteases and, therefore, called Subtilisin-like Proprotein Convertases (SPCs) or PCs. Seven members of this family have been identified and characterized and each have conserved signal peptides, pro-regions, catalytic and P-domains but differ in their C-terminal domains in mammals.
Autocatalytic cleavage of an N-terminal propeptide activates these proteases, which is required for folding, and activity also causes the release of prodomain. Other examples of function associated with proteolytic processing are the blood clotting cascades, the metaloendopeptidases, the secretases and the caspases. Yet other examples are the viral proteases that specifically process viral polyproteins.
The art describes various strategies to inhibit the various proteases. For instance, gamma-secretase inhibitors are presently being developed for the treatment of T cell acute lymphoblastic leukemia (Nature Medicine 2009, 15:50-58). Caspase inhibitors are being developed for a variety of different applications (The Journal of Biological Chemistry 1998, 273:32608-32613), for instance, in the treatment of sepsis (Nature Immunology 2000, 1:496-501).
A problem with the use of protease inhibitors is that these proteins typically have a range of targets in the human body and, associated therewith, a range of effects. Inhibiting a protease in the human body through the action of a protease inhibitor thus, not only inhibits the desired effect, but typically also has a range of other effects that may or may not affect the utility of the protease inhibitor for the indicated disease. Another problem associated with protease inhibitors is that it is not always easy to produce an inhibitor that is sufficiently specific for the target protease and, therefore, may also inhibit other proteases.